The present invention relates to a solid electrolyte fuel cell (a solid oxide electrolyte fuel cell: hereinafter referred to as an SOFC) using a solid electrolyte to obtain an electrical energy through electrochemical reaction and its related manufacturing method and, more particularly, to an SOFC equipped with an intermediate layer composed of thin film layers alternately laminated in an area between an electrode layer and a solid electrolyte layer, and its related manufacturing method.
The SOFC is structured to include a solid electrolyte, having ion conductivity properties such as an oxygen ion or proton, which is intervened between an oxidizing electrode and a reducing electrode and serves as a battery which allows oxidation gas containing oxygen gas to be supplied to the oxidizing electrode and reducing gas containing hydrogen or hydrocarbons to the reducing electrode whereby these gases react with one another through electrochemical reaction by means of the solid electrolyte.
In general, for the solid electrolyte, stabilized zirconia composed of zirconia (ZrO2) added with yttria (Y2O3) or scandia (Sc2O3), ceria (CeO2) system materials, Bi2O3 system materials or lanthanum gallate (LaGaO3) system materials with a perovskite structure is used. It is important for the solid electrolyte to have a performance not to allow electrons to be conducted but to allow ion transfer and, in a case where oxygen ion is a charge carrier, it is desired for the solid electrolyte to have a high conductance characteristic as to the oxygen ion. Also, another important characteristic of the solid electrolyte layer involves a gas impermeable property.
For the oxidizing electrode, it is a general practice to use metallic based materials such as silver (Ag) or platinum (Pt) and oxide material with the perovskite structure represented by LaSrMnO or LaSrCoO. The oxidizing electrode is required to have a characteristic that has high oxidation resistant property, high oxygen permeable property and a high electric conductivity, and also provides an excellent property of catalysis that converts oxygen molecules to oxygen ions.
Further, for the reducing electrode, it is a general practice to use materials such as nickel (Ni) or cermet composed of nickel and the solid electrolyte. The reducing electrode is required to have a characteristic that is resistant to a reducing atmosphere, a high reducing gas permeable property and a high electric conductivity, and also provides an excellent property of catalysis that converts hydrogen molecules to protons.
That is, the SOFC is structured with the component layers composed of respective films (layers) having such characteristics set forth above.
More particularly, the SOFC has a structure in which the solid electrolyte layer is kept between the reducing electrode layer and the oxidizing electrode layer, i.e. a structure laminated with three materials having different constituents. And, in such a structure, there are some instances where the coefficients of thermal expansion of the respective layers are different from one another.
Japanese Patent Application Laid-Open Publication No. H5-121084 discloses a structure in which the ratio of metallic materials forming a reducing electrode layer is continuously varied toward the outside from a portion corresponding to a surface of a solid electrolyte layer to remove a constituent boundary surface between the electrolyte layer and the reducing electrode layer.
U.S. Pat. Nos.5,753,385 and 6,007,683disclose a structure in which, in order that a difference in a coefficient of thermal expansion between a solid electrolyte layer and respective electrode layers is minimized, a mixed layer of electrolyte material and electrode material is formed as an intermediate layer between an electrolyte layer and reducing and oxidizing electrode layers.